1. Field of the Invention
The present invention relates to arrangements for feeding controlled amounts of powder into a gas stream, and more particularly to powder feeders for supplying controlled amounts of powder in plasma spraying systems.
2. History of the Prior Art
In plasma spraying systems some means must be provided for feeding controlled amounts of powder into a gas stream used to form the plasma spray. The powder which is typically of a metallic composition is typically heated prior to being fed into a passing gas stream at a relatively constant rate so that the spraying of the powder onto a workpiece within the plasma system is relatively uniform.
It is very important for most applications of a plasma spraying system that the feeding of powder into the gas stream be accomplished at a constant rate so that the quantity of powder within the gas remains relatively uniform. Variations in the supply of powder to the gas stream result in spraying nonuniformities and generally in difficulty in controlling the plasma spraying operation. The uniformity in the supply of the powder must be maintained in the face of varying factors such as variations in the quantity of powder stored within the powder feeder and the tendency of the powder to cake or sometimes harden under varying conditions of humidity so as to fail to flow freely and evenly into the apparatus which feeds it into the gas stream.
One type of powder feeder commonly employed in plasma spraying systems utilizes a hollow, generally cylindrical cannister mounted at approximately a 45.degree. angle and adapted to receive and store a quantity of powder therein. A rotating wheel at the lower end of the cannister is at least partially disposed within the cannister so as to load powder into a succession of slots spaced about the outer periphery of the wheel with the help of a tamper for periodically contacting the slots. As the wheel continues to rotate, the slots filled with powder are moved into the path of a gas stream such that the powder within the slots is entrained into the stream being directed through the slots.
Powder feeders of the type just described have been found to have a number of shortcomings which hamper their ability to provide a continuous, uniform supply of the powder into the gas stream. Thus, even though the powder is tamped into the slots in the wheel, at least some of the powder frequently falls out of the slot before the slot can be moved from the supply of powder at the bottom of the cannister to the path of the gas stream. Moreover, even where the successive slots remain substantially completely filled with powder, the areas of the wheel between adjacent slots periodically move into the path of the gas stream so as to interrupt the supply of powder to the gas stream. The height of the powder within the cannister plays a definite role in the uniformity of powder feeding, even when other factors are favorable. Loading of the powder into the slots may be hampered by varying conditions of humidity and other factors which may cause the powder to cake or harden or otherwise refuse to flow smoothly and evenly into the slots.
Accordingly, it would be desirable to provide a powder feeder capable of providing a relatively uniform supply of powder into a gas stream, even in the face of varying conditions. Such a feeder should lend itself to relatively simple driving and control. Furthermore, such feeder should be capable of providing a relatively uniform and continuous supply of powder from a storage vehicle for the powder in the face of variations in humidity and other conditions.